The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
In computer networks such as the Internet, packets of data are sent from a source to a destination via a network of links (communication paths such as telephone or optical lines) and nodes (usually routers directing the packet along one or more of a plurality of links connected to it) according to one of various routing protocols.
In the network, some nodes represent end systems (such as printers, fax machines, telephones, PC's etc) whereas other nodes represent network devices (e.g. switches, routers etc). The data packets are sent around the network from a source to a destination in accordance for example with routing information shared among the nodes of the network. As the network comprises a plurality of interconnected nodes, the network is fairly robust. Should a node or link fail for any reason, the network dynamically configures to re-route data so as to avoid the failed node. When a node or link comes into existence on a network, for instance through repair or by addition of a new node, the network dynamically converges to a so-called converged state wherein all the routers of the network have common routing information.
One class of routing protocol relying on shared information is the link state protocol. Examples of link state protocols are the Intermediate System-to-Intermediate System (IS-IS) protocol and the Open Shortest Path First (OSPF) protocol. The link state protocol relies on a routing algorithm resident at each node. Each node on the network advertises, throughout the network, links to neighboring nodes and provides a cost associated with each link which can be based on any appropriate metric such as link bandwidth or delay and is typically expressed as an integer value. A link may have an asymmetric cost, that is, the cost in the direction AB along a link may be different from the cost in a direction BA.
Based on the advertised information in the form of a link state packet (LSP) each node constructs a link state database (LSDB) which is a map of the entire network topology and from that constructs generally a single optimum route to each available node A link can be thought of as an interface on a router. The state of the link is a description of that interface and of its relationship to its neighboring routers. A description of the interface would include, for example, the IP address of the interface, the mask, the type of network it is connected to, the router connected to that network and so on. The collection of all these link-states for the whole network forms the link-state database. Link state protocols use a link state algorithm to build and calculate the shortest path to all known destinations. The algorithms themselves are quite complicated but the following provides a high level simplified way of looking at the various steps of a link state algorithm. Upon initialization or due to any changing routing information, a router will generate a link state advertisement packet (LSP). This advertisement represents the collection of all link states on that router. All routers exchange LSPs by means of flooding. Each router that receives a link state update, stores a copy in its link state database and then propagates the update to other routers with a propagation time of the order of 20 ms. After the database of each router is completed, the router will calculate the shortest (lowest cost) path tree to all designations and use this information to form an IP routing table. In some instances two or more routers of equal cost present themselves, termed an “equal cost path split”.
One appropriate algorithm is a shortest path first (SPF) algorithm. As a result a “spanning tree” is constructed, rooted at the node and showing an optimum path including intermediate nodes to each available destination node. Conversely, a “reverse spanning tree” can be constructed showing the optimum path to a given node from all nodes from which it is reachable. Because each node has a common LSDB (other than when advertised changes are propagating around the network) any node is able to compute the spanning and reverse spanning tree rooted at any other node. The results of the SPF are stored in a routing table (also known as a routing information base (RIB)) and, based on these results, the forwarding information base (FIB) or forwarding table is updated with an update time of the order of 500 ms to control forwarding of packets appropriately.
In link state protocols, when a link or a node fails and is subsequently repaired, or there is some other change to the network such as a change of link cost, the routers involved with the repaired part of the network then have to re-establish convergence. This is achieved by the router(s) advertising themselves or the change throughout the network area. However during topology change there will be a short period of time in which LSDBs, RIBs and, critically, FIBs across a network become inconsistent as information about a change is propagated through the network. Routes generated during this period of inconsistency may result in routing loops, which persist until the databases have converged (at which point there should be no loops, by definition). As an example, if a first node sends a packet to a destination node via a second node, comprising the optimum route according to the first node's SPF, a situation can arise where the second node, according to it's SPF (based on a different LSDB from that of the first node) determines that the best route to the destination node is via the first node and sends the packet back. The loop can happen where the first node, according to its LSDB believes that a link cost is lower than the second node does, according to its LSDB. This can continue indefinitely although usually the packet will have a maximum hop count after which it will be discarded. Such a loop can be a direct loop between two nodes or an indirect loop around a circuit of nodes. Re-convergence will typically take several hundred milliseconds and hence may cause disruption for periods greater than that originally caused by the failure.
One solution for avoiding loops during a routing transition is described in co-pending patent application Ser. No. 10/323,358, filed 17 Dec. 2002, entitled “Method and Apparatus for Advertising a Link Cost in a Data Communications Network” of Michael Shand (hereinafter “Shand”), the entire contents of which are incorporated by reference for all purposes as if fully set forth herein. According to the solution put forward in Shand, when a node detects deactivation of an adjacent link or node, then instead of advertising the failure of the component, for example by simply removing the link from the LSP, the node that detects deactivation increments the associated link costs and advertises the incremented cost. As a result even when nodes have different LSDBs because of finite propagation and processing time of the LSP carrying the incremented link cost, loops are not set up in the remainder of the network. Once all nodes have updated their LSDBs, the detecting node increments the cost and advertises the incremented cost again. However in some circumstances it is desirable to converge on a common view of a network more quickly than is permitted by this incremental approach.
One alternative approach to dealing with link failure is described in document “Fortifying OSPF/IS-IS Against Link-Failure” by Mikkel Thorup (“Thorup”) which is available at the time of writing on the file “If_ospf.ps” in the directory “˜mthorup\PAPERS” of the domain “research.att.com” on the World Wide Web. The approach of Thorup is to pre-compute the SPF at each node for each possible link failure. When a link failure is advertised the node forwards along its pre-computed updated path whilst updating the LSDB in the background.
Various problems arise with the approach. Thorup requires increased storage and computing to deal with all possible routes around all possible failures, as well as extra forwarding code requirements. Significantly Thorup does not address the problem of loop formation during a transition. A further solution for avoiding loops during a routing transition is described in co-pending patent application Ser. No. 10/442,589, filed May 20, 2003, entitled “Method and Apparatus for Constructing a Transition Route in a Data Communications Network” of Stewart Bryant et al. (“Bryant”), the entire contents of which are incorporated by reference for all purposes as if fully set forth herein. According to the solution put forward in Bryant, when a network component fails, upstream nodes construct transition routes to destinations which would otherwise be reachable via the failed component. The transition routes are constructed by tunneling packets for the destination node to an intermediate node in an intersection of a first set of nodes reachable from the repairing node without traversing the failed component and a second set of nodes from which the destination node is reachable without traversing the first component. Although this is also a very effective solution, it requires installation of a large number of tunnels.
Based on the foregoing, there is a clear need for a method of preventing loops occurring in a network during convergence of the network.